1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), and more particularly, to an LCD driving scaler which is capable of reducing electromagnetic interference.
2. Description of the Related Art
High-speed personal computers (PCs), which operate at high clock frequencies, are susceptible to a serious problem of electromagnetic interference (EMI). Display devices, such as large-sized monitors or LCDs, also have the same problem as the high-speed PCs because of a high pixel clock frequency. For this reason, a variety of research have been carried out with respect to methods for reducing EMI.
In order to reduce EMI, a metal shielding technique can be applied. Alternatively, a passive device, such as a multi-layered printed circuit board, a choke coil, or a bead, can be used. However, EMI is reduced through repetitive trials and failures, and thus increases in the material and manufacturing costs and the time taken to develop a product are inevitable.
In the meantime, growing attention has been paid to one EMI reduction method: a spread spectrum modulation method. According to the spread spectrum modulation method, the frequency of an input clock is modulated so that the clock frequency varies periodically.
FIGS. 1A and 1B are graphs showing frequency spectra respectively prior to and following frequency modulation performed for the purpose of reducing EMI.
Referring to FIGS. 1A and 1B, as a result of frequency modulation, the frequency spectrum of a clock is spread out over a wide range of frequencies, and, consequently, the maximum amplitude of the clock decreases. In general, a spread spectrum clock generator (SSCG), which is a frequency modulator capable of varying an input clock periodically, is used for spread spectrum modulation.
There are two different types of spread spectrum modulation techniques. One is a center spreading technique in which the frequency of a clock signal is modulated so that the frequency of the clock signal periodically varies about a center frequency in upward and downward directions by the same amount, and the other is a down spreading technique in which the frequency of a clock signal is modulated based on a lower frequency than a center frequency so that the frequency of the clock signal can be prevented from exceeding the center frequency.
FIG. 2 is a diagram illustrating a center spreading technique by which a triangular modulation profile is provided through frequency modulation. There are various modulation profiles provided by spread spectrum modulation techniques, such as, a triangular modulation profile, a sinusoidal modulation profile, and a so-called “Hershey-Kiss” modulation profile. Hereinafter, the modulation rate and the modulation period will be described in the following paragraphs with reference to FIG. 2, taking the triangular modulation profile as an example.
In FIG. 2, the modulation rate represents the width of the variation of the frequency of a modulated output signal obtained by modulating the frequency of an input clock signal in a spread spectrum modulation method, and the modulation period represents the period of the variation of the frequency of the modulated output signal. The modulation frequency is a reciprocal number of the modulation period.
An LCD monitor having SXGA resolution or higher also requires the above-described spread spectrum modulation technique using a spread spectrum clock generator because an LCD monitor having a high resolution uses a high-frequency system clock of about 100 MHz, which means that a user of the LCD monitor is susceptible to exposure to strong electromagnetic waves at such a high frequency level.
In general, spread spectrum modulation techniques, in which the frequency spectrum of an input system clock input into a scaler is spread by using a spread spectrum clock generator, have been applied to LCDs. Hereinafter, among the conventional spread spectrum modulation methods using a spread spectrum clock generator, two spread spectrum modulation methods using a spread spectrum clock generator before and after using a phase locked loop (PLL) will be briefly described.
In a conventional spread spectrum modulation method in which a spread spectrum clock generator is used prior to the PLL, the frequency of a clock signal obtained by performing spread spectrum on an input high-frequency system clock is divided before being processed by the PLL, and then a scaler pixel clock signal is generated in the PLL.
Here, the spread spectrum clock generator receives a system clock from a crystal oscillator, receives information necessary to control a modulation rate via input pins, and performs spread spectrum on the system clock according to a modulation frequency fixed at about 30–50 kHz.
On the other hand, in another conventional spread spectrum modulation method in which a spread spectrum clock generator is used following the PLL, the frequency of a high-frequency system clock is divided, and the result of the division is input into the PLL. Then, a scaler pixel clock signal is generated by spread-spectrum-modulating a signal output from the PLL.
Pixel data output in synchronization with the scaler pixel clock are provided to an LCD source driver via a gamma correction circuit so that a screen can be displayed on an LCD panel.
However, since the above-mentioned conventional spread spectrum modulation methods use a PLL in a spread spectrum clock generator and a PLL included in a scaler, a mismatch in frequencies between the two PLLs is more likely to occur. In other words, due to a mismatch in frequencies between a scaler output clock and a pixel driving clock, the scaler output clock fails to drive pixels. This problem can be solved by increasing the frequency division rate and thus reducing the phase difference between the two PLLs, but a high frequency division rate causes another problem as described below.
Assume that the modulation rate of a spread-spectrum-modulated clock signal is A and the frequency division rate is as high as 1000. Then, as a result of frequency-dividing the spread-spectrum-modulated clock before processing it in a PLL, the modulation rate for the clock signal to be input into the PLL is decreased to be as small as A/1000, which indicates a weak spread spectrum effect.
Spread spectrum clock generators used in the prior art have been manufactured by many companies, including Pulse Core Corp., ICS Corp., and Cypress Semiconductor Corp. In such spread spectrum clock generators, a modulation frequency is determined in advance by an input clock frequency, and only the modulation rate can be adjusted within several percent of the input clock frequency by an IC pin setting. Accordingly, it is impossible to set the modulation frequency to be the same as or a predetermined number of times higher than the frequency of an input horizontal synchronization signal HSYNC of a video signal. Therefore, it is impossible in this configuration to match the frequency of the input horizontal synchronization signal HSYNC with the modulation frequency. In addition, since pixel data are transmitted to vertical lines of an LCD panel at different moments in time, horizontal lines of the LCD panel have corresponding different brightnesses.
In the conventional approaches, since a spread spectrum clock generator is provided external to a scaler, it is impossible to perform a spread spectrum modulation technique to a clock signal within the scaler. In order to solve this problem, a spread spectrum clock generator can be provided immediately following a PLL included in a scaler so that the frequency spectrum of a clock processed in the PLL can be spread. However, in such a case, problems as a frequency mismatch between two PLLs, a weak spread spectrum effect, and a difference in brightness between lines of an LCD panel still remain unsolved.
In addition, in the conventional approaches, since a spread spectrum clock generator is provided external to a scaler, the scaler requires additional input/output pins for the spread spectrum clock generator, which results in an increase in the chip size.